The invention relates to a receiver and in particular to a UWB receiver that receives signals that represent a plurality of sub-carriers modulated by transmit data.
Ultra-wideband (UWB) is a radio technology that transmits data at very low energy levels over a wide bandwidth. UWB may be used for short-range, high-bandwidth communications such as those in personal area networks. Conventional UWB devices are permitted to use the spectrum from 3.1 GHz to 10.6 GHz, although in practice conventional UWB transmissions are limited to between 3.1 GHz and 4.8 GHz for practical reasons. One way of transmitting data using UWB would be to spread a transmission over the entire 1.7 GHz available spectrum by means of spread spectrum techniques. However, implementing circuits that are capable of processing such a wideband signal is challenging. Instead, a multi-band approach has been developed in which the available spectrum has been divided into three sub-bands that each have a bandwidth of approximately 500 MHz, as shown in FIG. 1. FIG. 2 shows an example of how frames of data may be transmitted across the three different sub-bands. In this example, the first frame is transmitted over the first sub-band, the second frame is transmitted over the third sub-band, the third frame over the second-sub-band, the fourth frame over the first sub-band and so on. Each frame of data 201 is separated from the preceding frame by a guard interval 204. Each frame may include a cyclic prefix 202 inserted before the transmit data 203. The transmit data may include one or more symbols.
UWB systems may transmit data using orthogonal frequency division multiplexing techniques (OFDM) to transmit information on each of the sub-bands. OFDM offers high spectral efficiency, is resilient to RF interference and is able to efficiently capture multi-path energy. An OFDM signal is typically a composite of a number of orthogonal sub-carriers modulated with baseband data. Each sub-carrier may be independently modulated using some type of phase-shift keying or quadrature amplitude modulation, or a combination of the two. The composite baseband signal is then used to modulate a main RF carrier for transmission.
Part of an OFDM transmitter is shown in FIG. 3. This section of the transmitter receives a single input data stream 301 that is converted into N parallel data streams 303 by means of a switching unit 302. A mapping unit may also be provided for mapping the parallel bit streams into parallel symbol streams, but this is not shown in FIG. 3. Each of the parallel data streams represents a signal in the frequency domain. An inverse Fourier transform is performed by processing block 304 to transform the parallel data streams into real and imaginary time domain signals that can be used to modulate an RF carrier signal 307.
Part of an OFDM receiver is shown in FIG. 4. This section of the receiver receives a signal and mixes it down to baseband 401 to generate real 402 and imaginary 403 data streams. These data streams represent a time domain signal and are input into a processing block 404 that performs a Fourier transform to recover N parallel data streams 405 in the frequency domain. A switching unit 406 receives the N parallel data streams and forms a single data stream 407 therefrom. Symbol demapping may also be performed at this point.
UWB transmitters typically use 128 sub-carriers or tones on each sub-band. Each symbol to be transmitted may be mapped onto a point on a modulation constellation for a particular sub-carrier. This point indicates the combination of phase and/or modulation to be applied to that sub-carrier. If N sub-carriers are used, and each sub-carrier is modulated by M alternative symbols, the OFDM symbol alphabet consists of MN combined symbols. The time domain signal can then be calculated as:
                                          v            ⁡                          (              t              )                                =                                    ∑                              k                =                0                                            N                -                1                                      ⁢                                          X                k                            ⁢                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      kt                    /                    T                                                                                      ,                  0          ≤          t          <          T                                    (        1        )            where v(t) is the time domain signal, {Xk} are the data symbols, N is the number of sub-carriers and T is the OFDM symbol time.
The original frequency domain signal can be recovered by calculating the Fourier transform of the time domain signal:
                                          X            k                    =                                    ∑                              t                =                0                            T                        ⁢                                          v                ⁡                                  (                  t                  )                                            ⁢                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      kt                    /                    T                                                                                      ,                  0          ≤          k          ≤                      N            -            1                                              (        2        )            UWB receivers may encounter difficulties when one or more of the sub-carriers in the sub-band being used is subject to interference. This situation is shown in FIG. 5, which shows a UWB signal that is obscured by interference over some of its frequency spectrum. This interference can cause the output of the FFT to saturate for the affected sub-carriers. If the interference is extensive, it might even cause either or both of the in-phase and quadrature components generated by mixing the received signal down to baseband to saturate. Existing receivers deal with saturated sub-carriers by simply setting the FFT output for that sub-carrier to the maximum magnitude possible. However, if the reason for the saturation is that the saturated sub-carriers were subject to interference, then the interference will influence the decoding of the signal and may cause bit errors. Therefore, there is a need for an improved UWB receiver that is capable of limiting the impact that interference has on the decoding of a received signal.